The present invention relates to nontoxic gas generating compositions which upon combustion, rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to nonazide gas generants that produce combustion products having not only acceptable toxicity levels, but that also exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures. Additionally, the compositions of the present invention readily ignite and sustain combustion at burn rates heretofore thought to be too low for automotive airbag applications.
The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the discussions of which are hereby incorporated by reference.
In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.
One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. The solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate.
It is known that the use of ammonium nitrate as an oxidizer contributes to the gas production with a minimum of solids. To be useful, however, gas generants for automotive applications must be thermally stable when aged for 400 hours or more at 107.degree. C. The compositions must also retain structural integrity when cycled between -40.degree. C. and 107.degree. C.
Generally, gas generant compositions using ammonium nitrate are thermally unstable propellants that produce unacceptably high levels of toxic gases, CO and NO.sub.x for example, depending on the composition of the associated additives such as plasticizers and binders. Known ammonium nitrate compositions are also hampered by poor ignitability, delayed burn rates, and significant performance variability. Several prior art compositions incorporating ammonium nitrate utilize well known ignition aids such as BKNO.sub.3 to solve this problem. However, the addition of an ignition aid such as BKNO.sub.3 is undesirable because it is a highly sensitive and energetic compound.
Yet another problem that must be addressed is that the U.S. Department of Transportation (DOT) regulations require "cap testing" for gas generants. Because of the sensitivity to detonation of fuels often used in conjunction with ammonium nitrate, most propellants incorporating ammonium nitrate do not pass the cap test unless shaped into large disks, which in turn reduces design flexibility of the inflator.
Accordingly, many nonazide propellants based on ammonium nitrate cannot meet requirements for automotive applications. Two notable exceptions are disclosed in U.S. Pat. No. 5,531,941 in which the use of phase-stabilized ammonium nitrate, triaminoguanidine nitrate, and oxamide is taught, and, in U.S. Pat. No. 5,545,272 in which the use of phase-stabilized ammonium nitrate and nitroguanidine is taught. Despite their usefulness in automotive applications, these compositions are still problematic because triaminoguanidine nitrate and nitroguanidine are explosive fuels that complicate transportation requirements and passing the cap test. Furthermore, because of poor ignitability and a relatively low burn rate, the nitroguanidine composition requires a conventional ignition aid such as BKNO.sub.3 which is both sensitive and very energetic.
Certain gas generant compositions comprised of ammonium nitrate are thermally stable, but have burn rates less than desirable for use in gas inflators. To be useful for passenger restraint inflator applications, gas generant compositions generally require a burn rate of at least 0.40 ips (inches/second) at 1000 psi. In general, gas generants with burn rates of less than 0.40 ips at 1000 psi do not ignite reliably and often result in "no-fires" in the inflator wherein only a portion of the gas generant is combusted. Poor ignitability, even with complete combustion, results in a gas production rate too slow for automotive airbag applications.